There is a popular German game board and card games site, http://www.brettspielwelt.de/Spiele/ (available in English). I used to play some games there and it seem like a rich picking ground for next AI challenge.

Specifically, there'a a card game named Tichu and a board game named Dog. What makes them interesting even more is that you play in teams (2 x 2), even though you are not explicitly allowed to communicate with your partner. I think teamplay would be interesting to program.